This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Many innovations have improved the television experience over the past few years, as for example the number of programs, the digital treatment, the definition of the pictures and the way to broadcast programs to the users over some new types of networks. However during this period the television experience has also suffered from specific drawbacks like the channel change time. Today it may take sometimes several seconds to change the display from one channel to another one, when it was nearly instantaneous with the analog television system in the past. This drawback is particularly true for IPTV and mobile systems but is generally impacting all digital broadcasting systems, regardless of the medium they are carried upon.
Video distribution via broadband or broadcast networks uses standard compression systems such as MPEG-2 (ISO/IEC 13818) or JVT/H.264/MPEG AVC. The MPEG standards define the GoP (Group of Pictures). The GoP defines a sequence of I, P, B type pictures. These pictures are also defined in the MPEG standards. The I picture (Intra) is encoded without reference to any other picture. The P (Predictive) picture is described with respect to preceding pictures. The B (Bidirectional) picture is described with respect to preceding and following pictures. When a receiver receives a program, it waits for the reception of an I picture to start decoding. This causes a delay prior to the display of a new film, particularly in the case of zapping. Hence, when the user changes the channel, time passes before display of the new channel on the screen. This time is divided between the the time to receive a decodable I picture in the stream and the time to fill the reception memory.
According to MPEG video compression standards, some time stamps are transmitted from the encoder to the decoder. The transport stream carrying the audio/video program includes some program clock reference information (PCR) which carries samples of system time clock (STC) used by the encoding system. The time information is used to synchronize the clock system in the decoder. The elementary stream for an audio/video program carries some time information called presentation time stamps (PTS) which indicate the time when the audio/video data should be available in the decoder for the rendering of the program. Thus, a PTS indicates the value of the PCR when the video content of a program is displayed by the decoder.
Several solutions exist in order to improve the channel change time, by adding an additional stream whose structure is such that a decodable picture is more rapidly found to be able to display it. This is what is defined for example in the patent application WO2005112465A1. An improvement of this solution to reduce the channel change time is known by introducing a delay between the main stream and the additional stream in order for the decoder to receive a decodable picture still earlier.
The figures FIG. 3, FIG. 4 and FIG. 5. illustrate some channel change solutions already known. The stream is a sequence of GoPs. The presentation time stamp information (PTS) associated with the I-picture is shown in the figures and the successive program clock reference (PCR) values indicate the time when the pictures are displayed.
FIG. 3 illustrates a standard channel change method in a television system using a video compression standard such as MPEG. From the channel change command, the receiver waits for the next I picture in the program stream and then decodes and displays the incoming pictures from the moment when the program clock reference (PCR) fits the presentation time stamp associated with the I-picture. The time interval between the channel change command and the next I picture is called Random Access Point (RAP). The time interval between the first I-picture received and the value of the PCR to display is called ΔPCR/PTS. Finally the time between the channel change command and the start of the rendering is RAP+ΔPCR/PTS. By using this solution, the channel change delay may be important.
FIG. 4 illustrates an optimized channel change method in a television system using a video compression standard as MPEG by using a secondary program stream delivered to the receiver in parallel of the main program stream. The secondary stream is encoded in a lower resolution than the main stream and the GOP size of the secondary stream is reduced with respect to the GOP size of the main stream. The receiver then receives the next I picture earlier compared to a channel change method where only one program stream is transmitted. The time from the channel change command to the rendering of the program is reduced compared to the channel change solution described in FIG. 3.
FIG. 5 illustrates an optimized channel change method in a television system using a video compression standard as MPEG by using a secondary program stream delivered to the receiver in parallel of the main stream with a delay between the main program stream and the secondary program stream. Thus the next I picture associated with a given presentation time stamp is received later according to the solution presented in FIG. 4 by the receiver and the time in the reception buffer is smaller than without delay. The time ΔPCR/PTS is reduced. This optimizes the channel change time compared to the solution described in FIG. 4.
However, an additional stream always requires the use of additional bandwidth to carry a program and this is a drawback for the transmission.